We have previously characterized a model for immune activation of HIV-1 in which viral proteins as well as infectious virus are induced in HIV-1 transgenic (Tg) mice by infection with intracellular protozoan and bacterial pathogens commonly encountered by AIDS patients. Viral expression in this system appears to be restricted in situ and in vitro to antigen presenting cells (APC) with T cells failing to respond even after polyclonal stimulation. In this years work we examined the role of toll like receptor signalling in the microbial induction of HIV-1 expression in our Tg mouse model. Microbial ligands known to specifically activate TLR-2, TLR-4, and TLR-9 were shown to trigger p24 protein production by Tg spleen cells and macrophages and when combined did so in additive fashion. To explore the role of TLR signalling in HIV gene expression induced by live pathogens, we constructed HIV-1-Tg mice deficient in TLR2 (Tg/TLR2(-/-)) or the downstream adaptor molecule MyD88 (Tg/MyD88 (-/-) by crossing wild-type Tg mice with the appropriate knock-out animals. Spleen cells from both Tg/TLR2(-/-) and Tg/MyD88 (-/-) mice were found to be completely defective in p24 production induced in response to live M. tuberculosis or Mycobacterium avium as well as certain mycobacterial products. Interestingly, while totally impaired in the Tg/MyD88 (-/-) spleen cell cultures, TNF production was only partially diminished in the Tg/TLR2(-/-) cultures suggesting that HIV-1 and proinflammatory cytokine induction involve distinct pathways. Importantly, following in vivo mycobacterial infection, Tg/TLR2(-/-) mice failed to display the enhanced HIV-1 gag/env mRNA and p24 protein synthesis exhibited by wild-type Tg animals thus confirming a unique role for this TLR signaling pathway in immune activation vivo. Together, these results argue that activation of HIV-1 expression by mycobacterial coinfections is critically dependent on an MyD88 dependent signaling pathway specifically involving TLR-2. This pathway may represent a potential target for intervention in TB-HIV, a disease in which AIDS progression appears to be excacerbated by the simultaneous presence of Mycobacterium tuberculosis.